The present invention relates to a method and to apparatus for determining the purity of a metal alloy.
More precisely, the invention relates to determining whether the concentration of impurities in a metal alloy, and in particular the concentration of oxides that result from making the alloy is unacceptable, acceptable, or very low so as to be able to determine whether the alloy in question is usable or not. This problem arises in particular, but not exclusively, with aluminum alloys.
These impurities that result from making the alloy, in particular an aluminum alloy, may consist essentially in oxides. Nevertheless, it is also possible to find salts, carbides, nitrides, borides, and sludge in the form of particles or of a kind of skin that forms on the alloy.
It will be understood that it is necessary, or at least desirable, once an alloy has been made, to ensure that it does not include too high a content of impurities, which could make use of the alloy difficult or impossible for making certain types of part subjected to high mechanical stresses.
Various methods have been used for determining impurity content. These various methods comprise: spectroscopic emission; various types of chemical analysis, e.g. gas chromatography; another type of measurement is volumetric analysis using centrifuging or filtering techniques; and then there are non-destructive techniques, such as using ultrasound or techniques based on using X-rays.
In the filter group of techniques, mention can be made in particular of the ALCAN method known under the name PODFA which is based on the following principles: about 2 kg of alloy are taken directly while in molten form. This alloy to be tested is caused to pass through a filter. The residue in the filter is then selected vertically along the central plane and prepared for metallographic analysis. The analysis makes it possible to determine the concentration of impurities or inclusions expressed in mm.sup.2 /kg. To cause the alloy to pass through the filter, it is necessary to apply air at sufficient pressure onto the free surface of the alloy in the receptacle which contains it.
However, it will be understood that that filter-based technique requires subsequent metallographic analysis, which takes a certain amount of time, and which therefore means that genuinely instantaneous analysis cannot be provided while the alloy is being made.